c# - 如何解决错误“对象引用未设置为对象的实例。”

Question

• 这是我的代码

   public class Bill 
  {
     public int BillNumber { get; set; }
     public DateTime BillDate { get; set; }
     public List<BillLine> LineItems { get; set; }
  
  
  
   public void AddBillLine(BillLine billLine)
   {
       LineItems.Add(billLine);
   }
  
   public void RemoveBillLine(int SOMEID)
   {
       throw new NotImplementedException();
   }
  
   /// GetTotal returns the sum of (Cost * Quantity) for each line item
  
   public decimal GetTotal()
   {
       throw new NotImplementedException();
   }
  
   public void MergeBill(Bill sourceBill)
   {
       throw new NotImplementedException();
   }
  
   /// deep clone of the current bill (all fields and properties)
  
   public Bill Clone()
   {
       throw new NotImplementedException();
   }
  
   public override string ToString()
   {
       throw new NotImplementedException();
   }
  }
  
  
  
    public class BillLine
  {
   public int BillLineId { get; set; }
   public string Description { get; set; }
   public int Quantity { get; set; }
   public double Cost { get; set; }
   }
  
  
  
  
     public class Program
   {
      static void Main(string[] args)
   {
       Console.WriteLine("Billing app started....");
  
       CreateBillWithOneItem();
       CreateBillWithMultipleItemsAndQuantities();
       RemoveItem();
       MergeBill();
       CloneBill();
       BillToString();
   }
  
   private static void CreateBillWithOneItem()
   {
       var bill = new Bill();
  
       bill.AddBillLine(new BillLine()
       {
           BillLineId = 1,
           Cost = 6.99,
           Quantity = 1,
           Description = "Apple"
       });
  
       Console.WriteLine(bill.GetTotal());
   }
  
   private static void CreateBillWithMultipleItemsAndQuantities()
   {
       var bill = new Bill();
  
       bill.AddBillLine(new BillLine()
       {
           BillLineId = 1,
           Cost = 10.21,
           Quantity = 4,
           Description = "Banana"
       });
  
       bill.AddBillLine(new BillLine()
       {
           BillLineId = 2,
           Cost = 5.21,
           Quantity = 1,
           Description = "Orange"
       });
  
       bill.AddBillLine(new BillLine()
       {
           BillLineId = 3,
           Cost = 5.21,
           Quantity = 5,
           Description = "Pineapple"
       });
  
       Console.WriteLine(bill.GetTotal());
   }
  
   private static void RemoveItem()
   {
       var bill = new Bill();
  
       bill.AddBillLine(new BillLine()
       {
           BillLineId = 1,
           Cost = 5.21,
           Quantity = 1,
           Description = "Orange"
       });
  
       bill.AddBillLine(new BillLine()
       {
           BillLineId = 2,
           Cost = 10.99,
           Quantity = 4,
           Description = "Banana"
       });
  
       bill.RemoveBillLine(1);
       Console.WriteLine(bill.GetTotal());
   }
  
   private static void MergeBill()
   {
       var bill1 = new Bill();
  
       bill1.AddBillLine(new BillLine()
       {
           BillLineId = 1,
           Cost = 10.33,
           Quantity = 4,
           Description = "Banana"
       });
  
       var bill2 = new Bill();
  
       bill2.AddBillLine(new BillLine()
       {
           BillLineId = 2,
           Cost = 5.22,
           Quantity = 1,
           Description = "Orange"
       });
  
       bill2.AddBillLine(new BillLine()
       {
           BillLineId = 3,
           Cost = 6.27,
           Quantity = 3,
           Description = "Blueberries"
       });
  
       bill1.MergeBill(bill2);
       Console.WriteLine(bill1.GetTotal());
   }
  
   private static void CloneBill()
   {
       var bill = new Bill();
  
       bill.AddBillLine(new BillLine()
       {
           BillLineId = 1,
           Cost = 6.99,
           Quantity = 1,
           Description = "Apple"
       });
  
       bill.AddBillLine(new BillLine()
       {
           BillLineId = 2,
           Cost = 6.27,
           Quantity = 3,
           Description = "Blueberries"
       });
  
       var clonedBill = bill.Clone();
       Console.WriteLine(clonedBill.GetTotal());
   }
  
   private static void BillToString()
   {
       var bill = new Bill()
       {
           BillDate = DateTime.Now,
           BillNumber = 1000,
           LineItems = new List<BillLine>()
           {
               new BillLine()
               {
                   BillLineId = 1,
                   Cost = 6.99,
                   Quantity = 1,
                   Description = "Apple"
               }
           }
       };
  
       Console.WriteLine(bill.ToString());
   }
  

  }

• 谁能建议如何解决此错误：System.NullReferenceException：'对象引用未设置为对象的实例。'

• 任何建议我需要在我的应用程序中做哪些必要的更改以避免错误

• 错误来自 LineItems.Add(billLine);

例如。尝试添加

List<BillLineItem> BillLineItems = new List<BillLineItem>();

public void AddBillLine(BillLine billLine)
    {
        LineItems.Add(billLine);
    }

标签： c#.net.net-corec#-4.0console-application

Answer 1

您收到null reference异常的原因是由于Bill课程中的这一行：

public List<BillLine> LineItems { get; set; }

它目前没有初始化列表。最简单的解决方法是：

public List<BillLine> LineItems { get; set; } = new List<BillLine>();